Synthesis of phosphonium-substituted vinylidene complexes from [HC≡CCH2PPh3]+: Exploring the Competition between Allene and Vinylidene Formation.

Reaction of the alkenyl-substituted carbene complex [Ru(κ2-O2CMe)(OC{Me}O(═CCH═CH2)(PPh3)2]+ with PPh3 results in the formation of the vinylidene-containing species [Ru(κ1-O2CMe)(κ2-O2CMe)(═C═CHCH2PPh3)(PPh3)2]+, among other products. This complex could also be obtained by reaction of [Ru(κ2-O2CR)(PPh3)2] (R = Me, Ph) with [HC≡CCH2PPh3]+ although in this instance competitive formation of allene-containing [Ru(κ1-O2CR)(κ2-O2CR)(η2-H2C═C═CHPPh3)(PPh3)2] occurred. Reaction of [Ru(κ2-O2CMe)2(PPh3)2] with [H2C═C═CHPPh3]+ gives this allene complex in a selective fashion. Similar results were obtained when [RhCl(PiPr3)2]2 was used as the metal precursor. The reaction with [H2C═C═CHPPh3]+ gave [RhCl(η2-H2C═C═CHPPh3)(PiPr3)2]+, whereas treatment with [HC≡CCH2PPh3]+ afforded a mixture of the same complex and [RhCl(η2-HC≡CCH2PPh3)(PiPr3)2]+, which subsequently isomerized to vinylidene-containing [RhCl(═C═CHCH2PPh3)(PiPr3)2]+ via the alkynyl hydride complex [RhCl(H)(C≡CCH2PPh3)(PiPr3)2]+. Consistent with previous fin...